The present invention relates to a data transmission device for a loop transmission system which is applicable to a local area network or the like.
A loop transmission system employs two independent loops which allow signals to flow therethrough in opposite directions to each other, taking into account possible failures in the transmission paths. Generally, when a failure occurs in one of the loops which is in operation, it is switched to the other loop to continue communications.
One approach to the recovery from a failure in a loop is disclosed in "32 Mb/s Optical Fiber Loop Network: H-8644", HITACHI REVIEW, Vol. 31, No. 3, June 1982, pages 125-130. In the system disclosed, when a loop service node (LSN), or master station, detects a loop timing failure, it interrupts the delivery of loop timing which is occurring in the transmission paths of an active loop and, while feeding out a particular pattern to the transmission paths of the other or back up loop, switches the loop from the active to the back up. Meanwhile, all the field service noded (FSN), or slave stations, check a signal in the back up loop and switch the loop when they have found a loop timing failure in the active loop. When both the loops have failed, the LSN sends out a command for sequentially looping back remote FSNs to thereby cause the FSNs into loop back operation and, awaiting a return of the loop timing, locates a failure portion to set up a loop back path which excludes the failure portion.
This type of prior art system has the drawback that once the LSN or master station causes a failure therein, the whole system becomes down making the loop unable to be reconstructed. Another drawback is that the LSN has to memorize various factors related to the FSNs such as the number and sequence. Additionally, an intricate procedure is required for coping with the loop back condition.